Fluid flow control devices, such as a control valves and regulators, are commonly used to control fluid flowing through a pipe. A typical fluid control device, such as a control valve, includes a valve body defining an inlet, an outlet, and a fluid flow path extending between the inlet and the outlet. A valve seat within a seat ring may be coupled to the body to define an orifice and closure surface within the flow path. A throttling element, such as a valve plug, is moveable relative to the seat ring to control fluid flow through the orifice. Certain fluid flow control devices employ internal components, such as a cage, which may guide movement of the valve plug in the control valve and may characterize fluid flow between the inlet and outlet. The cage generally defines an interior bore sized to receive the throttling element and typically includes at least one passage through which the fluid flow path passes. The throttling element is moveable between an open and a closed position in which the throttling element modulates or controls the fluid flow relative to the seat ring. In the closed position, the throttling element engages the valve seat within the seat ring, typically positioned at a distal end of the cage, to substantially prevent fluid flow through the valve. It is generally understood that the valve seat, and therefore the seat ring, preferably aligns within the throttling element and matches its concentricity to provide fluid tight closure or shutoff.
Conventional fluid flow control devices employ various methods for retaining the seat ring within the valve body and aligning the seat ring with the throttling element. One such method for retaining the seat ring uses a threaded engagement between a seat ring and a valve body. That is, an outer surface of the seat ring may be threaded such that the seat ring may be screwed into a corresponding threaded surface within the valve body along the flow path. To affect a seal between the seat ring and the interior surface of the valve body, a substantial amount of torque must be applied to the seat ring during assembly. The necessary amount of torque generally increases exponentially as the diameter of the port (i.e. the diameter of the orifice) increases. However, the large torque applied to the seat ring in such a design can result in radial distortion of the seat ring that may compromise the seal between the valve body, the seat ring and the throttling element, thereby reducing or degrading the shutoff capability of the valve.
Moreover, it can be difficult to apply the required torque to the screwed-in seat rings to provide an acceptable seal. That is, the location of the seat ring with respect to the internal flow paths may make accessing the seat ring difficult. Additionally, special tools are typically required for assembly of the screwed-in seat ring in the valve body. These difficulties also extend to removal of the screwed-in seat rings for repair and/or replacement. Repair and/or replacement of the seat ring may be further complicated by the relatively high contact stresses between the screwed-in seat ring and the valve body that may damage the threaded engagement at the valve body when the seat ring is installed.
In another method for installing conventional seat rings in a fluid flow control device, a seat ring may be directly bolted into a valve body to secure the seat ring in place. That is, the seat ring may be fabricated with through-holes about the periphery of the seat ring to receive bolts that secure it to the valve body. The bolt-in seat ring typically requires multiple tappings in the valve body for receiving the bolts. Because the bolts attaching the seat ring are in tension, high strength materials are required to fabricate the fluid flow control device. In some devices, the high strength bolting requirements limit the acceptable material choices to more expensive materials such as the nickel-based alloy Inconel 718 available from Specialty Metals of Kokomo, Ind. Similar to screwed-in seat rings, high bolt torques are required to retain the seat ring in the valve body and may be difficult to apply to bolts located down inside the valve body. The high bolt torque may also increase the possibility of seat ring distortion (i.e. making the seat ring substantially non-planar and/or non-axial) that may result in leakage between the seat ring and the valve body, or between the seat ring and the throttling element. Additionally, bolts in tension may be more susceptible to stress-corrosion cracking.
In other examples, a seat ring may be welded to an interior wall of a valve body. Control valves having welded-in seat rings are expensive to fabricate and install. In many cases, the valve body must be spun on a vertical lathe to machine the seat ring or special tooling is required to machine the seat while the valve body stays stationary. Either manufacturing method is expensive to implement and very expensive to repair.
Anther method for retaining a seat ring within a fluid flow control device is to provide a clamping element, such as a cage or seat ring retainer, to clamp the seat ring in place. These conventional clamping elements can add significant expense to the fluid flow control devices over other devices that do not secure the seat ring in such a manner. Moreover, where the seat ring, the clamping element and/or the valve body are fabricated from different materials, a differential thermal expansion between the valve body and the clamping element can significantly limit the operating temperature range of the fluid flow control device. Additionally, different temperature zones resulting from variable material thickness within the valve body can further exacerbate differential thermal expansion. One typical solution to prevent leakage due to differential thermal expansion is to fabricate the valve body, seat ring and clamping element from materials with similar coefficients of thermal expansion. However, this may result in adding significant cost to valve.
Further, a clamped seat ring typically requires a gasket between the seat ring and the valve body to provide a fluid seal therebetween. The gasket loading force must originate at the body-to-bonnet bolting and be transferred through the bonnet to the cage to the seat ring to load the gasket. The necessary force needed to form the seal at the gasket can require larger body-to-bonnet bolts, additional material within the valve body web, and thicker flanges at the inlet and outlet of the valve—all of which increase the cost of the control valve.
In large flow control devices, for example a control valve having a port size or seat ring cross-sectional area of at least six inches in diameter, it is generally understood that maximizing port size is critically important in increasing fluid flow capacity (i.e., the flow capacity of the valve is directly proportional to the square of the port area). To accommodate larger seat rings for increased flow capacity for a given fluid flow device body, the opening or head of the fluid flow device may have to be increased in diameter to receive the larger seat ring, which causes an increase in bolting requirements as previously discussed.
Another method to increase the port size relates to maximizing the seat ring opening or port. To maximize the port area, the seat ring may be made “thinner” by removing material about the periphery of the seat ring to enable the seat ring to pass into the head of the valve body for a given valve size while removing material from the interior of the seat ring to increase the orifice diameter. As the seat ring becomes thinner, it may become more susceptible to distortion when the seat ring is tightened down onto the valve body using any of the methods described above. Seat ring distortion is a primary contributor to fluid flow control device leakage, which can lead to trim damage (e.g. high velocity flows that may cause plug or seat erosion in high pressure applications) in the device. It is also more difficult to affect a satisfactory seal between large seat rings and their respective/receiving bodies.
In view of the existing methods for retaining seat rings within fluid flow control devices, and the operating requirements and ranges for the devices, a need exists for an improved seat ring retention mechanism and method that allow the fluid flow control devices to be manufactured easier, potentially with reduced cost and without the need for special tools or machining processes, and that facilitate the repair and replacement of the seat rings when necessary. Further, the need exists for an improved seat ring retention mechanism that securely retains the seat ring within the body of the device without causing distortion of the seat ring and the accompanying leakage issues, even in larger fluid flow control devices.